To a wide extent, for electro-optic indicator elements, the properties of nematic or nematic-cholesteric liquid crystalline materials are employed. These include significant changes in their optical properties, such as light absorption, light scattering, birefringence, reflectivity or color under the influence of electric fields. The function of such indicator elements thereby depends, for example, on the phenomena of dynamic scattering, the deformation of aligned phases, the Schadt-Helfrich effect in the twisted cell or the cholesteric-nematic phase transition.
For the technical use of these effects in electronic constructional elements, liquid crystalline dielectrics are needed which must satisfy a number of requirements. Especially important is chemical stability towards moisture, air and physical influences, such as heat, radiation in the infra-red, visible and ultra-violet ranges and direct and alternating electric fields. Furthermore, there is required of technically useable liquid crystalline dielectrics a liquid crystalline mesophase in the temperature range of at least +10.degree. C. to +50.degree. C., preferably of 0.degree. C. to 60.degree. C., and the lowest possible viscosity at room temperature, which is preferably not more than 70.times.10.sup.-3 Pa.s. Finally, in the range of visible light, they are to have no inherent absorption, i.e., they must be colorless.
A number of liquid crystalline compounds is already known which satisfy the stability requirements demanded of dielectrics for electronic constructional elements and which are also colorless. Included, in particular, are the p,p'-disubstituted benzoic acid phenyl esters described in published Federal Republic of Germany Patent Application No. 2,139,628 and the p,p'-disubstituted phenylcyclohexane derivatives described in published Federal Republic of Germany Patent Application No. 2,636,684. In both mentioned classes of compounds, and also in other known series of compounds with a liquid crystalline mesophase, there is no individual compound which, in the required temperature range of 10.degree. C. to 60.degree. C., forms a liquid crystalline nematic mesophase. Therefore, as a rule, mixtures of two or more compounds are prepared in order to obtain substances useable as liquid crystalline dielectrics. For this purpose, one usually mixes at least one compound with low melting or clear point with another with distinctly higher melting and clear point. A mixture is thereby normally obtained, the melting point of which lies below that of the lower melting component, and whose clear point lies between the clear points in the components. Nevertheless, the preparation of optimal dielectrics still causes difficulties since the components with the high melting and clear points frequently also impart a high viscosity to the mixtures. The operating times of the electro-optical indicator elements produced using them are thereby prolonged in an undesirable manner. Furthermore, problems arise since often the solubility of the various components in one another, especially at room temperature or lower temperatures, is only very limited.